1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus for imaging organs within a living body and so on by transmitting and receiving ultrasonic waves to generate ultrasonic images to be used for diagnoses.
2. Description of a Related Art
In medical fields, various imaging technologies have been developed for diagnoses by observing inside of an object to be inspected. Especially, ultrasonic imaging for acquiring interior information of the object by transmitting and receiving ultrasonic waves enables image observation in real time and provides no exposure to radiation unlike other medical image technologies such as X-ray photography or RI (radio isotope) scintillation camera. Accordingly, ultrasonic imaging is utilized as an imaging technology at a high level of safety in a wide range of departments including not only the fetal diagnosis in obstetrics, but gynecology, circulatory system, digestive system, and so on.
The principle of ultrasonic imaging is as below. Ultrasonic waves are reflected at a boundary between regions with different acoustic impedances like a boundary between structures within the object. Therefore, by transmitting ultrasonic beams into the object such as a human body, receiving ultrasonic echoes generated within the object, and obtaining reflection points where the ultrasonic echoes are generated or reflection intensity, outlines of structures (e.g., internal organs, diseased tissues, and so on) existing within the object can be extracted.
The acoustic impedance is a constant intrinsic to a material as expressed by equation (1) or (2), and the unit of MRayl (mega Rayl) is generally used therefor and 1Mrayl=1×106 kg·m−2·s−1.Z=ρ·C  (1)Z=(ρ·K)1/2  (2)where “ρ” represents density of an acoustic medium, “C” represents acoustic velocity within the acoustic medium, and “K” represents a bulk modulus of the acoustic medium.
Further, given that the acoustic impedance of the first medium is Z1 and the acoustic impedance of the second medium adjacent to the first medium is Z2, the vertical reflectance “R” of ultrasonic waves at the interface between the first medium and the second medium is given by the following equation (3).R=(Z2−Z1)/(Z2+Z1)  (3)
Generally, an ultrasonic image is generated based on the intensity of ultrasonic waves reflected at the respective sampling points within the object, but an attempt to obtain information within the object based on phases of ultrasonic waves has been made. Especially, when Z1>Z2, the reflectance R is negative and the phases of ultrasonic waves are inverted, and therefore, the attempt is considered to be effective for acquiring tissue properties within the object.
As related technologies, Japanese Patent Application Publication JP-A-11-113893 discloses an ultrasonic diagnostic apparatus including complex signal converting means for converting a reception signal obtained by transmitting and receiving ultrasonic waves into a complex signal, and image forming means for forming an ultrasonic image based on only one of a real part and an imaginary part of the complex signal.
Japanese Patent Application Publication JP-A-11-113894 discloses an ultrasonic diagnostic apparatus including complex signal converting means for converting a reception signal obtained by transmitting and receiving ultrasonic waves into a complex signal, phase difference calculating means for calculating phases of respective sampling points on an ultrasonic beam from the complex signal to obtain phase differences between the respective sampling points by phase comparison, and variance calculating means for calculating a variance of the phase differences.
Japanese Patent Application Publication JP-A-11-113895 discloses an ultrasonic diagnostic apparatus including complex signal converting means for converting a reception signal obtained by transmitting and receiving ultrasonic waves into a complex signal, phase difference calculating means for calculating phases of the respective sampling points from the complex signal, and phase display means for displaying the phases.
Japanese Patent Application Publication JP-A-11-113896 discloses an ultrasonic diagnostic apparatus including complex signal converting means for converting a reception signal obtained by transmitting and receiving ultrasonic waves into a complex signal, ratio calculating means for calculating a ratio between a real part and an imaginary part of the complex signal, and display means for displaying the ratio between the real part and the imaginary part.
Japanese Patent Application Publication JP-A-11-137546 discloses an ultrasonic diagnostic apparatus including complex signal converting means for converting a reception signal obtained by transmitting and receiving ultrasonic waves into a complex signal, and phase difference calculating means for calculating phases of respective sampling points on an ultrasonic beam from the complex signal to obtain phase differences by phase comparison between the sampling points, wherein an ultrasonic image representing properties of living body tissues based on the phase differences is displayed.
In the above-mentioned documents, the complex signal is obtained by quadrature detection processing of the reception signal, however, the documents do not disclose display of the phase information in an easy-to-understand way based on the results. Further, the amount of information of the complex signal in the low-frequency band is small because it is sampled at a lower sampling rate than that for the original reception signal and the noise contained in the complex signal affects thereon, and accordingly, there is a problem that the phase calculation accuracy can not be obtained sufficiently.